1. Field of the Invention
The present invention relates to a process for the separation and recovery of a purified 3,4-toluenediamine product from an ortho-toluenediamine isomer mixture containing 2,3- and 3,4-toluenediamines. Furthermore, the present invention simultaneously relates to a process for producing a mixture of 4- and 5-methylbenzotriazoles from this 2,3- and 3,4-toluenediamine mixture.
2. Description of the Prior Art
Toluene diisocyanate (TDI), a valuable intermediate in the preparation of polyurethanes, is generally produced today by a multi-step process. This widely used process includes the steps of dinitrating toluene to produce a mixture of ortho- and meta-dinitrotoluene isomers, reducing this isomer mixture to form a mixture of ortho- (i.e., 2,3- and 3,4-, and meta- (i.e., 2,4- and 2,6-) toluenediamine isomers, separating the desired meta-isomers from the by-product ortho-isomers, and finally reacting these meta-isomers with phosgene to produce toluene diisocyanate. The separation of the desired 2,4- and 2,6-toluenediamine isomers from the by-product ortho-isomer fraction has been normally carried out by fractional distillation as disclosed in U.S. Pat. Nos. 3,637,514 and 3,732,239, both issued to Spatz et al on Jan. 25, 1972 and May 8, 1973, respectively, or by crystallization techniques in the presence of a selected solvent as disclosed in U.S. Pat. No. 3,149,162, issued to Gardner et al on Sept. 15, 1964.
Several million pounds of this ortho-toluenediamine by-product fraction is produced annually in the United States. But, because only limited amounts of this ortho-isomer mixture can be converted into commercial products such as corrosion inhibitors in antifreezes and the like, a substantial amount of this by-product must be disposed of, usually by burning.
In recent years, it has been discovered that relatively pure amounts of 3,4-toluenediamine may be employed in the synthesis of various heterocyclic compounds such as benzimidazoles, quinoxalines, and phenazines, whereas the above-mentioned commercially available ortho-toluenediamine isomer mixture cannot be so employed. Therefore, those persons working in this art have been desirous of a practical process that will separate the 2,3- and 3,4-isomers (sometimes referred to hereinafter as 2,3-TDA and 3,4-TDA) and then separately utilize them. Such a process could greatly benefit the overall TDI manufacturing operation since it would open up commercial uses for by-product, much of which must now be burned.
One process, disclosed in U.S. Pat. No. 3,960,963, issued to Gavin on June 1, 1976, does disclose a crystallization process for separating 2,3- and 3,4-toluenediamine isomers. In particular, this patented process dissolves a mixture of the isomers in either benzene, toluene, or xylene. While this patent does disclose a process which produces a very pure 3,4-toluenediamine product, there are presently no known uses for the 2,3-isomer product left in the organic solvent, and therefore, normally this 2,3-isomer product/organic solvent mixture is usually disposed of by burning. Furthermore, there are other practical difficulties with this Gavin process. For example, strict environmental regulations today normally prevent the large-scale burning of organic solvents like benzene, toluene or xylene. Also, the cost of burning these solvents will usually make the over-all process uneconomical. Moreover, the evaporation of the 2,3-isomer from these organic solvents is impractical since this isomer when out of solution decomposes readily and thus is unusable.
As mentioned in the Gavin patent, crystallization techniques are generally only used where it is desired to separate a pure component from a mixture containing the component and a minor amount of an impurity (usually 5% by weight of the mixture or less). Normally, however, where the impurity constitutes more than a minor amount of the mixture, one skilled in the art would not expect crystallization to be an effective tool to separate the desired component from the mixture. Thus, it would be expected that the use of crystallization techniques would not produce a desired degree of separation. Furthermore, it would be surprising that a widely used non-organic solvent such as water would be effective to dissolve aromatic compounds like these ortho-toluenediamine isomers as much as it has been found to by the present invention. Thus, it was quite unexpected and surprising to find that when commercially available ortho-toluenediamine isomer mixtures which contain about 40% to about 70% by weight 3,4-toluenediamine and about 30% to about 60% by weight 2,3-toluenediamine was dissolved in water and cooled, a substantially pure 3,4-toluenediamine crystalline product was obtained in high yields.